The present invention relates to apparatus, systems, and methods for the catalytic removal of ozone from air, and more particularly, to the removal of ozone from an air stream supplied to an interior air space.
Conventional commercial aircraft feed bleed air from a gas turbine engine to an environmental control system (ECS) and thence to an interior air space, e.g., cabin or flight deck of the aircraft. The ECS conditions the air it receives in terms of pressure, temperature, and humidity to provide for the comfort of flight crew and passengers. However, the ECS does not remove pollutants, such as ozone, from the air stream supplied to the aircraft cabin and flight deck.
Modern jet (gas turbine engine) aircraft are typically designed for fuel-efficient operation at relatively high altitudes of 25,000 feet or more where the atmospheric ozone content is relatively high. The ozone concentration may depend on a number of factors, such as the altitude, geographic location, time of year, etc. The ozone concentration in the atmosphere is typically in the range of from about 0.2 to 2.0 ppm. The upper limit permitted by FAA regulations for the ozone concentration in cabin air of commercial aircraft is 0.1 ppm. Excessive levels of ozone can cause a number of medical problems, including lung and eye irritation, headaches, fatigue, and breathing discomfort.
In the prior art, catalytic converters have used palladium as a catalyst to reduce the concentration of ozone in the bleed air from the engines to an acceptable level. The bleed air temperature on most aircraft systems is above 300° F. Newer aircraft may use dedicated ambient air compressors to provide an air stream, to be fed to the ECS, at a temperature substantially less than 300° F. (for example, from about 100 to 200° F.). However, the efficiency of palladium for the catalytic removal of ozone decreases at temperatures below about 300° F. In addition, palladium is reversibly deactivated when exposed to lower temperatures, e.g., less than about 300° F. For example, the catalytic activity of palladium may decline during exposure to a constant temperature in the range of from about 100 to 300° F.
Certain prior art catalytic converters for the removal of ozone have used alumina as a catalyst support. Alumina is susceptible to the deactivation of palladium catalyst thereon, for example, by poisoning from sulfur- and phosphorus-containing compounds. In an attempt to protect palladium supported on alumina from phosphorus induced catalyst deactivation, palladium has been combined with a transition metal, such as Ni or Mn, as a co-catalyst.
However, the vulnerability of catalysts to sulfur-containing contaminants may increases when transition metals are used, because transition metals may form sulfates.
U.S. Pat. No. 5,080,882 to Yoshimoto et al., discloses a catalyst structure and method for ozone decomposition, wherein the catalyst structure comprises a thin carrier material having a catalyst supported thereon. The catalyst may include a zeolite (aluminosilicate) containing Cr, Zn, V, W, Fe, Mo, Ni, Co, Ru, Cu, Rh, Pd, Ag, or Pt, or an oxide thereof.
U.S. Pat. No. 5,422,331 to Galligan et al. discloses a layered catalyst composition having a refractory metal oxide underlayer and a refractory metal oxide overlayer on a metal substrate, together with one or more catalytic metal components dispersed on the overlayer. The catalytic metal components may include a palladium component and a manganese component. A chloride ion scavenger, such as silver oxide, may be used to prevent corrosion of the metal substrate.
U.S. Pat. No. 6,214,303 to Hoke et al. discloses the removal of various pollutants from the atmosphere by passing air over a stationary substrate. Catalyst materials may be supported on a refractory metal oxide. Useful catalyst compositions disclosed by Hoke et al. for the decomposition of ozone include MnO2, Mn2O3, CuO, Carulite®, carbon, Pd, and Pt.
EP 0233642 A2 to Cornelison et al. discloses a process for hydrogenation of organic compounds (fats and oils) using a hydrogenation catalyst. The catalyst comprises a metal substrate, a washcoat on the substrate, and a catalytic metal on the washcoat. The washcoat may be alumina, titania, silica, magnesia, or a zeolite; and the catalytic metal may be Pd, Pt, Ni, Cu, Ag, or mixtures thereof.
As can be seen, there is a need for a catalyst system and method for ozone removal from an air stream being supplied to an interior air space, wherein the catalyst system includes a low-temperature catalyst in combination with a high-temperature catalyst, such that the system can operate efficiently at both a relatively low temperature, e.g., less than 300° F., and a relatively high temperature, e.g., 300° F. and above.
There is a further need for a catalyst system and method for ozone removal from an air stream over an extended service period, wherein the catalyst system is resistant to deactivation by contaminants, such as S- and P compounds, and wherein a catalytic composition of the catalyst system does not require the inclusion of a transition metal. There is still a further need for a catalyst system for the effective removal of ozone from an air stream, wherein the system can be operated over an extended period of time without the need to replace or service the catalyst system.